The present invention relates to laser beam direction and modulation and more specifically to a method and apparatus including an integrated laser beam splitter and modulator providing higher speed and increased efficiency.
Laser beams are often modulated for various uses. One example is direct imaging technology, such as a laser direct imaging (LDI) device for a printed circuit board (PCB) panel. LDI may be performed by scanning a laser beam across the surface of a PCB panel from one edge of the PCB panel to the other edge, along one or more scan lines. For examples of LDI systems and their use, see U.S. Pat. No. 5,895,581 to Grunwald (issued Apr. 20, 1999) entitled LASER IMAGING OF PRINTED CIRCUIT PATTERNS WITHOUT USING PHOTOTOOLS, and U.S. Pat. No. 5,328,811 to Brestel (issued Jul. 12, 1994) entitled METHOD OF PRINTING AN IMAGE ON A SUBSTRATE PARTICULARLY USEFUL FOR PRODUCING PRINTED CIRCUIT BOARDS. See also co-pending U.S. patent application Ser. No. 09/435,983 to Vemackt, et al. (filed: Nov. 8, 1999) now U.S. Pat. No. 6,396,561, entitled: METHOD AND DEVICE FOR EXPOSING BOTH SIDES OF A SHEET, assigned to the assignee of the present invention and incorporated herein by reference for all purposes. See also an automatic material handling system for a LDI device that is described in U.S. patent application Ser. No. 09/511,646 to Vernackt (filed Feb. 22, 2000) now U.S. Pat. No. 6,387,579 entitled A SYSTEM, METHOD AND ARTICLE OF MANUFACTURE FOR DIRECT IMAGE PROCESSING OF PRINTED CIRCUIT BOARDS, and assigned to the assignee of the present invention.
Because laser beam sources can be very complex and expensive, often a device that uses a laser beam source also divides or in some other manner splits the laser beam into one or more laser beams. Each separate laser beam can then be used for a separate application. In some applications multiple laser beams are required to accommodate a higher throughput of the system. For example, often the modulation speed of a laser beam is a limiting factor. To overcome the limited modulation speed, the laser beam is split into multiple laser beams. Each one of the multiple laser beams is then modulated with a portion of the modulation signal, thereby providing multiple, parallel channels of modulation data throughput. In other applications, the limiting factor may be a mechanical limitation. An example of a mechanical limitation is a rotating drum upon which the laser is used to create an image. To increase the throughput, the laser beam is divided into multiple laser beams and each of laser beams are applied to a portion of the drum, resulting in a faster imaging process on the drum.
An LDI device described above is typically used to apply an image to PCB panels. The PCB panels are coated with a photoresist material (photoresist). The photoresist can be any one of several materials well known in the art, for example Riston(copyright) Photoresist (E. I. du Pont de Nemours and Company, Research Triangle Park, N.C.) or Laminar(copyright) Photoresist (Morton Electronic Materials, Tustin, Calif.). In the industry it is believed that for a given photoresist, a given quantity of light energy E must be imparted to the photoresist to properly and completely expose or react the photoresist. This has been expressed in the form of a product of power of the light source and exposure time as expressed in Equation 1:
E=Ixc3x97txe2x80x83xe2x80x83Equation 1 
Where:
I=intensity of the UV light (mW/cm2)
t=time of exposure (seconds)
E=energy (mJ/cm2)
1W=1 J/s
Several types of lasers may be suitable as a laser light source for exposing photoresist in a direct imaging process or similar imaging processes such as a photoplotter. A commonly used laser is a continuous wave (CW) ultraviolet (UV) laser having a relatively low power of 1 to 4 watts. Such lasers are typically UV gas-ion lasers, and are available from, for example, Coherent, Inc., Santa Clara, Calif., and Spectra-Physics Lasers, Inc. Mountain View, Calif. Solid state UV CW lasers are also currently being developed. Solid state UV CW lasers also have relatively low laser power. Other lasers include a visible laser source, an infrared laser source, or an HeNe laser source. A mode locked laser source that provides a repetition rate that is equal to or higher than the modulation data pixel rate can be considered as a quasi CW laser source and therefore can also be used.
With the relatively low laser energy level that such lasers provide, efficient use of the laser power is required so that adequate light energy E is imparted to the surface material such as photoresist. Thus there is a need for a method and apparatus for efficient laser beam directing, splitting and modulating.
A combined modulator and laser beam splitter device is disclosed. The combined device includes a crystal with a horizontal cross-sectional shape of a pentagon. The crystal includes a top surface, a bottom surface, and a first through fifth sides. The first side and the second side are substantially parallel. An absorber is mounted on the third side. The fourth and fifth sides are substantially opposite to the third side. The fourth and fifth sides form an angle substantially equal to 180 degrees minus the sum of a first and second Bragg angles. The crystal also includes at least one layer. For one embodiment, each layer includes an incident window on the first side, an active window on the second side and a transparent axis between the incident window and the active window. A first and a second transducer are mounted on the fourth and fifth sides.
For one embodiment, the crystal can also include multiple crystals that are formed such that at least two of the multiple crystals are molecularly bonded.
For another embodiment, the multiple crystals may be mounted to mechanical mounts such that the crystals may be mechanically aligned to substantially minimize any optical imperfections.
For one embodiment, a first and a second acoustic wave direction intersect the absorbing surface at an angle substantially equal to 90 degrees minus the Bragg angle. For another embodiment, the first and the second acoustic wave directions meet at an angle substantially equal to twice the Bragg angle.
For one embodiment, each one of the output laser beams include a proportionate share of the energy of the incident laser beam.
For one embodiment, the third side is substantially perpendicular to the first and second sides. For another embodiment, the third side also forms an acute angle with the bottom side.